Overstimulation of glutamate receptors, especially the N-methyl-D-aspartate receptor (NMDAR), has been associated with neuronal cell injury and death following hypoxic-ischemic, neurodegenerative, and trauma-related insults that can lead to mental retardation and developmental disabilities. Previously, this Program Project Grant demonstrated that the NMDAR antagonist, Memantine, was a clinically-tolerated yet effective neuroprotectant because of its unique mode of action as an uncompetitive, low-affinity, rapid off-rate, open-channel blocker. Subsequently, Memantine, was shown to be clinically useful but only of moderate benefit. In Project II, we will test new, more effective NMDAR antagonists, the NO-Memantines, that are being developed in collaboration with Project I. These agents combine the channel blocking effect of Memantine with S-nitrosylation (targeted delivery of the NO group to critical cysteine thiols on the NMDAR to downregulate excessive activity). We will test these drugs for their ability to prevent hypoxic-ischemic neuronal damage. In these studies, we will attempt to ameliorate NMDAR-mediated neurotoxicity first in vitro and subsequently in vivo in rodent stroke models in a developmental fashion, in both neonates and in young adults. We will also perform extensive safety studies to ensure that we develop clinically-tolerated drugs since the NMDAR plays roles in both normal and abnormal brain activity. Additionally, we will study the effect on primary neurons of a new family of NMDAR subunits that was recently cloned by this P01, designated NR3A/3B. These subunits in some sense mimic the NMDAR antagonist drugs since expression of NR3 subunits in combination with the classical NR1/NR2 subunits decreases NMDAR activity, and may thus offer neuroprotection in early development when NR3A expression is predominant. Our proposed studies with novel NMDAR antagonists have important implications for the treatment of hypoxic-ischemic brain injury and various neurodegenerative disorders. Accordingly, the Specific Aims of Project II are the following: 1. To characterize the effects of NO-Memantines on excessive NMDA-evoked currents vs. physiological NMDAR-mediated synaptic activity (excitatory postsynaptic currents or EPSCs). 2. To test the ability of NO-Memantines to prevent NMDAR-mediated neurotoxicity in vitro and in vivo. 3. To study normal and pathophysiologically relevant actions of NR3A and 3B subunits in primary neurons in culture using patch-clamp recording techniques and neurotoxicity experiments.